Liquid neon, with more than 30 times the refrigerant capacity, per unit of volume, of liquid helium, is an economical cryogenic refrigerant. Neon is also used in the manufacture of electrical and electronic equipment, such as lightning arrestors, wave meter tubes, high-voltage indicators, television tubes and lasers.
Currently, there is an increased interest in the use of neon as a coolant. This interest arises from neon's physical properties, including: its low boiling temperature, which allows cooling to temperatures in the range 24.degree.-43.degree. K.; comparatively high gas and liquid density; high latent heat of evaporation; and noncombustibility. In addition, neon's status as an inert gas also provides advantages. All of these properties are similar to the corresponding properties of liquid hydrogen, with the exception of non-combustibility. Liquid hydrogen and hydrogen gas are highly combustible and pose a risk of fire or explosion. The use of liquified neon makes it possible to conduct experiments using temperature regimes similar to those of liquid hydrogen without the risk of explosion or fire.
One problem with the use of neon is that it is usually obtained as a mixture of helium and neon. Conventionally, it has been difficult and impractical to separate the neon and the helium to produce high purity neon on an industrial scale. Current commercial scale processes for the purification of neon from helium on an industrial scale require the use of complicated equipment including heat exchangers, rectification columns, and compressors.
One known apparatus for the separation of neon and helium using liquid hydrogen is disclosed in V. G. Fastovsky, A. E. Rovinsky and U. V. Petrovsky, "Inert Gases." At the boiling point of liquid hydrogen under atmospheric pressure (20.4.degree. K.), pure neon is in a solid state (the temperature of triple point for neon is 25.56.degree. K.) and helium is gaseous. However, when liquid hydrogen is at atmospheric pressure, the neon-helium mixture cannot be cooled below 20.4.degree. K. At that temperature the saturated vapor pressure of neon is 37.3 millimeters of mercury. As a result, there is a noticeable amount of neon impurities in the gaseous fraction, which lowers the quality of mixture separation and reduces the amount of neon recovered from the mixture. Creating a vacuum over the liquid hydrogen can lower its boiling temperature to 14.degree. K., which permits helium recovery with little neon contamination, and also permits the recovery of high purity neon (V. G. Fastovsky, A. E. Rovinsky, and U. V. Petrovsky "Inert Gases").
This apparatus provide a laboratory scale extraction of 0.067 nm.sup.3 of pure neon per hour. In this process, 200 liters of liquid hydrogen were required to obtain 1 nm.sub.3 of pure neon. This process uses a container cooled from the outside by liquid hydrogen. Because of its design, only a small quantity of neon can be frozen out at any one time. It is then necessary to stop the apparatus after solidifying a small amount of neon and to pump out the gaseous helium which contains a neon impurity. A fresh charge of the neon/helium mixture is then introduced into the vessel and additional neon is solidified. The residual helium and neon is then pumped out of the vessel again. This procedure is repeated until the vessel is completely filled with solid neon. At that time, an electric heater is used to melt and evaporate the neon. O. Tabunchikov et al, "Chem. Petrol. Eng." describes a similar process.
This process suffers from several obvious drawbacks. First, the use of electric heating elements in the presence of highly flammable liquid hydrogen or hydrogen gas is clearly undesirable. In addition, the process is very inefficient, requiring multiple charging and evacuation steps to obtain a small amount of purified neon. It is unsuitable for large-scale commercial application.
Another known apparatus which has been used to separate neon from helium is that described in USSR Inventors Certificate No. 1011144, 1981. This apparatus consists of two coils, provided with two heaters on their lower parts. The coils are located in a cryostat above the surface of the cryogenic liquid. For cooling purposes, each coil is in a shell which covers it from bottom to top. The coils are interconnected by pipes, with switch valves that permit transmission of the condensed mixture from the bottom of one coil to the top of the other.
This apparatus has several drawbacks which preclude its use in the large scale separation of neon from helium. First, the mixture which is introduced into this apparatus should have a solidification temperature (solid state phase transition/pass) which is higher than the boiling point of the cryogenic liquid that is in the cryostat. Thus, this apparatus can be used for the purification of neon from admixtures with solidification temperatures which are higher than liquid hydrogen's boiling temperature. However, this will not be effective for a neon-helium mixture, using liquid hydrogen as the cryogenic agent, as helium can not be condensed in such an apparatus.
It would be desirable to provide an apparatus and method for the separation of helium from neon which did not require continual cessation of the process to remove the helium from the container and which did not use electrical heating elements. It would also be desirable to provide an apparatus which would allow the simplified isolation of neon from helium on an industrial scale.
One object of the present invention is to provide an apparatus for the separation of neon and helium which produces highly purified neon on an industrial scale at an economical cost.
Another object of the present invention is to provide an efficient method for the production of purified neon, on large scale with high productivity and reliability.